The problem with the current wheels is they are a multi-stage type paint. This includes having a base colour then a less solid coat over them. Makes it have a lovely look - from dark grey to silver in some lights - but is a sod to match, as actually theres 2 colours to match - base colour and next colour - and there might even be colouring in the clear coat, so 3 colours to match! And you have to paint them in 3 stages too, no single touch up will ever match.
Best bet would be to go for a metallic anthracite colour. You get the flecks of metallic catching the light to give a good look, but its a lot easier to touch in when required as you can just use the single stage paint to do it (technically it should be 2 stage as it shoud be colour + clear coat, but you can mix them together for small touch ups).
Ask the wheel place what colour system they use. When I did mine with My Alloys in Basingstoke (on the 350z), they sent me down the road to Brown Brothers where they get their paint from and to have a look at their swatches. I took them all outside and matched them up to the car to get the colour I wanted. Your wheel place should be able to do the same